Code-excited linear prediction (CELP) is a well-known class of speech coding with good performance at low to medium bit-rates, 4 to 16, kb/s. CELP coders generally operate on fixed-length segments of an input signal called frames. A multimodal CELP coder is one that classifies each input frame into one of several classes, called modes. Modes are characterized by distinct coding techniques.
Typically, multimodal CELP coders include separate modes for voiced and unvoiced speech. CELP coders have employed various techniques to distinguish between voiced and unvoiced speech. These techniques, however, generally fail to properly characterize certain transient sounds as voiced speech. Another common problem in CELP coders is that the output speech gain does not always match the input gain.